The present invention relates to cleaning optical elements, particularly to cleaning surfaces having very demanding cleanliness requirements and, more particularly, to cleaning optical substrates prior to coating them with reflective coatings, such as extreme-ultraviolet (EUV) reflective coatings.
Recently, substantial effort has been directed to the development of reflective coatings, such as multilayer reflective coatings deposited on an optical substrate for various applications. Such multilayer structures are of particular technological importance in high-resolution, multiple-reflection imaging systems now being developed for projection lithography. Extreme-ultraviolet (EUV) lithography systems require several high precision optics coated with reflective multilayers. One of the problems in the fabrication of such high precision optics is in the preparation of the optical substrate for deposition of the multilayer thereon. Prior cleaning processes which utilize soap, spin-rinsing and drying have been utilized for optics cleaning. Also, prior cleaning processes which utilize freon or trichlorethylene have been developed which have produced satisfactory results. However, the use of freon or trichlorethylene in manufacturing areas has been restricted or eliminated due to environmental concerns. Thus, there has been a need for a cleaning process for EUV optical substrates, for example, which utilizes environmentally safe materials, but yet are effective in cleaning surfaces with very demanding cleanliness requirements.
The present invention provides a solution to the above-referenced optical surface cleaning by providing a process which satisfies the very demanding cleanliness requirements without the use of either freon or trichlorethylene. The cleaning process of the present invention uses ultrasonic cleaning in acetone, methanol, and a pH neutral soap, such as FL-70 soap, followed by rinsing in de-ionized water and drying with filtered nitrogen in conjunction with a spin-rinse.